1. Field of the Invention
Aspects of the present invention relate to a fusing device that fuses an image transferred to a print medium and an image forming apparatus employing the same, and more particularly, to a fusing device of an endless belt type and an image forming apparatus employing the same.
2. Description of the Related Art
In general, an electrophotographic image forming apparatus prints an image by scanning light to a photosensitive medium that is charged with a predetermined electric potential in order to form an electrostatic latent image, developing the image with a toner of a color, and transferring and fusing the image onto a print medium. A fusing device is provided on a printing passage of the image forming apparatus and fuses the toner image transferred to the print medium.
The fusing device is classified into a roller type and a belt type. In the roller type, a toner image is fused by a heating roller and a pressure roller. In the belt type, a contact surface between the print medium and an endless belt is wide as compared to a nip in a fusing device of the roller type. Accordingly, the fusing device of the belt type does not require an external diameter of the heating roller to be enlarged in order to increase the contact surface. Thus, a configuration of the fusing device becomes compact. Also, as the contact surface is relatively wide in the fusing device of the belt type as compared with that of the roller type, a fusing performance is relatively better. Also, a warm-up time is reduced and an image can be fused during high-speed printing.
FIG. 1 is a schematic cross-sectional view of a conventional fusing device of an endless belt type. Referring to FIG. 1, a conventional fusing device includes a heating roller 11, a pressure roller 21, a separation roller 31 that faces the heating roller 11, and a fusing pressure belt 15 that winds around the pressure roller 21 and the separation roller 31.
The pressure roller 21 and the separation roller 31 support the fusing pressure belt 15 such that the fusing pressure belt 15 rotates. Furthermore, the pressure roller 21 and the separation roller 31 allow a portion of the fusing pressure belt 15 to be in contact with the heating roller 11 or a print medium M being fed. The pressure roller 21 is elastically pressured along a line L1 going through the center O1 of the heating roller 11 and the center A1 of the pressure roller 21. Accordingly, the pressure roller 21 is elastically pressured in a perpendicular direction to a surface of the heating roller 11.
The separation roller 31 is elastically pressured along a line L2 going through the center O1 of the heating roller 11 and the center B1 of the separation roller 31. Accordingly, the separation roller 31 is elastically pressured in a perpendicular direction to a surface of the heating roller 11.
The lines L1 and L2 meet at the center O1 of the heating roller 11 as an apex and make a predetermined angle (i.e., the sum of θ1 and θ2). Thus, a distance between the center A1 of the pressure roller 21 and the center B1 of the separation roller 31 varies according to an amount of elastic pressure applied to the rollers 21 and 31. Given two optional lines P1 and P2 that go through the center A1 of the pressure roller 21 and the center B1 of the separation roller 31 and are parallel with each other, the line L1 inclines at θ1 to the line P1 and the line L2 inclines at θ2 to the line P2.
Here, a variation of the distance between the pressure roller 21 and the separation roller 31 is influenced by a condition of the print medium M that passes between the heating roller 11 and the fusing pressure belt 15. That is, if a relatively thick printing medium M or a plurality of sheets of print media M pass therebetween, the pressure roller 21 and the separation roller 31 move in a direction such that they become farther away from the heating roller 11. Accordingly, the distance between the pressure roller 21 and the separation roller 31 increases. As a result, a pressure applied to the fusing pressure belt 15 increases. Thus, the fusing pressure belt 15 may become deformed or slip, thereby causing an image distortion, an image offset, a print medium jam, and the like.